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Hydrogen trapping and quasi-cleavage fracture in Al-Zn-Mg alloy

Al-Zn-Mg合金の水素トラップと擬へき開破壊

都留 智仁   ; 清水 一行*; 山口 正剛   ; 板倉 充洋  ; 海老原 健一   ; Bendo, A.*; 松田 健二*; 戸田 裕之*

Tsuru, Tomohito; Shimizu, Kazuyuki*; Yamaguchi, Masatake; Itakura, Mitsuhiro; Ebihara, Kenichi; Bendo, A.*; Matsuda, Kenji*; Toda, Hiroyuki*

7xxx系合金は、析出硬化の主要な合金元素としてZnとMgを含む高強度Al合金である。これらの合金では、ごく一部の溶存水素が「擬へき開」破壊と呼ばれる異常な脆性破壊を起こすことが分かっている。本研究では、高強度Al-Zn-Mg合金の水素脆化に着目し、水素の平衡分配を推定した。実験観察と電子状態計算を組み合わせた理論モデルにより、Al合金の準劈開破壊に関連する前例のない水素脆化機構を探索した。第一原理計算により、空孔,刃状/らせん転位,粒界,$$eta$$-MgZn$$_{2}$$析出物などの様々な欠陥構造と水素との結合エネルギーを評価した。その結果、界面が完全にコヒーレントで界面周辺の自由体積が小さいにもかからわらず、Al-MgZn$$_{2}$$界面の結合エネルギーは粒界や空孔の結合エネルギーよりも大きいことがわかった。そこで、界面における凝集エネルギーと占有率の関係を評価した。水素の分配を総合的に検討することにより、Al-Zn-Mg合金の水素による擬へき開破壊のメカニズムを発見した。

7xxx series alloys are the high strength Al alloys which contain Zn and Mg as the major alloying elements for precipitate hardening. In these alloys, a small portion of dissolved hydrogen has been found to cause an anomalous brittle fracture called "quasi-cleavage" fracture. In the present study, we focus on the hydrogen embrittlement of high-strength Al-Zn-Mg alloys and estimate the equilibrium partitioning of hydrogen. We explored an unprecedented HE mechanism related to the quasi-cleavage fracture in Al alloys through experimental observation and theoretical model combined with electronic structure calculations. First-principles calculations were carried out to evaluate the binding energy between hydrogen and various defect structures such as vacancy, edge/screw dislocations, grain boundary, and $$eth$$-MgZn$$_{2}$$ precipitate. The results show that the binding energy at the Al-MgZn$$_{2}$$ interface is higher than that of the grain boundary and vacancy, even if the interface remains completely coherent and the free volume around the interface is small. The Al-MgZn$$_{2}$$ interface is therefore one of the most favourable trap sites among possible defect structures in Al alloys. Subsequently we evaluated the relationship between cohesive energy and occupancy at the interface. According to the comprehensive investigation of hydrogen partitioning, we advocate a new process of hydrogen-induced quasi-cleavage fracture in Al-Zn-Mg alloys.

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